1. Field of the Invention
The present invention relates to a liquid ejecting device including a head in which a plurality of liquid ejecting portions each having nozzles are arranged in parallel, and a liquid ejecting method using a head in which a plurality of liquid ejecting portions having nozzles are arranged in parallel. The present invention also relates to a technology that forms a pixel column or a pixel by deflecting droplets ejected from the nozzle of each liquid ejecting portion, and using a plurality of different liquid ejecting portions in adjacent positions.
2. Description of the Related Art
A method that uses an area ratio gray-scale method to represent an image has been known as a typical half-toning method in printing technology. In the area ratio gray-scale method, an image is decomposed into pixels of the minimized size and is represented by points of colors. A halftone gradation method and a dithering pattern gradation method are known as types of the area ratio gray-scale method. In the former, the diameters of dots having constant thickness are changed, while in the latter, dot density in a unit area is changed, with the dot diameter maintained to be constant.
Inkjet printers also use a method similar to the above area ratio gray-scale method. The method is divided into the following three types depending on the head structure of each inkjet printer.
FIG. 18 illustrates a method by superimposition, which is a first example of the related art. In FIG. 18, a head forms dots onto printing paper by ejecting droplets while moving in the arrow direction (the direction from left to right). At first, in the first movement (indicated by the dotted line in FIG. 18) of the head, the head forms dots a1 and a2 by ejecting droplets so that regions in which the dots a1 and s2 are formed can overlap with each other. In the second movement (indicated by the solid line in FIG. 18) of the head, the head forms dots a3 and a4 by ejecting droplets so that the dots a3 and a4 can respectively overlap with the dots a1 and a2 formed in the first movement and so that the dots a3 and a4, which are adjacent in the head moving direction, can overlap with each other.
As described above, one pixel composed of the four dots a1, a2, a3, and a4 is formed. This formation of one pixel from the four dots a1 to a4 can express five gradations, including the case of no dot. Also, by increasing the precision of the dot-formed positions in the first and second movements, a high quality image can be obtained.
FIG. 19 illustrates a method by droplet amount, which is a second example of the related art. In the second example, a head can switch the amount of droplets for ejection to three levels. The head forms a pixel by using any of a small dot b1, an intermediate dot b2, and a large dot b3. It is said that this method can increase printing speed.
FIG. 20 illustrates a method by the number of dots, which is a third example of the related art. In this method, dots c1, c2, . . . , whose diameters are smaller than a dot pitch are consecutively ejected. In addition, before a first formed dot is absorbed by (infiltrates) printing paper, the next dot is formed so as to, at least, overlap with the first delivered dot. In the example in FIG. 20, after the dot c1 is first formed, dots c2, c3, and c4 are sequentially formed before the dot c1 is absorbed by (infiltrates) the printing paper. This forms a larger dot c5 (in this case, dot c5 corresponds to one pixel).
The above examples of the related art have the following problems.
In the first example, the dots a1 to a4 must be formed in one pixel formation region a plural number of times (four times in the first example). Thus, a photograph or the like which has many gradations requires a longer printing time, compared with the case of printing a document. Also, although some number of gradations can be obtained, there is a limitation in increasing the number of gradations.
In the second example, it is difficult to accurately control the quantities of ejected droplets. This causes variations in the quantities of ejected droplets, and it is difficult to obtain stable image quality. Also, in order that plural types of droplet quantities may be ejected, the head structure becomes complicated, thus causing a high cost. Moreover, if droplet quantity can be changed, the number of types is limited to about three.
In addition, when the head has an ink ejecting portion that does not eject droplets, or an ink ejecting portion that ejects droplets of insufficient quantities, image quality deteriorates. Accordingly, printing using superimposition as in the first example must also be used. This causes a problem of a long printing time.
In the third example, after droplets are ejected once, a time is required to fill the ink ejecting portions with ink for the ejected ink. Thus, a certain amount of time is needed until re-ejection of droplets. Specifically, for example, a certain amount of time is required from ejection of the droplet for forming the dot c1 to ejection of the droplet for forming the dot c2.
As a result, during a movement of the head in one line in a serial method, in one pixel formation region, it is difficult to form the dots c2, c3, and c4 by delivering droplets before the formed dot c1 is absorbed by (infiltrates) the printing paper. Also, the movement speed of the head is very small when the head is moved so that, after the ink ejecting portions are filled with ink, in one pixel formation region, the dots c2, c3, and c4 can be formed before the formed dot c1 is absorbed by (infiltrates) the printing paper. Accordingly, this case is not practical.
As described in the first example and the third example, a method that forms one dot a5 so that the dots a1 to a4 overlap with one another, and a method that forms one dot c5 so that the dots c1 to c4 overlap with one another are characteristic in a serial method in which the head ejects ink droplets while moving back and forth in a line direction (the direction perpendicular to the traveling direction of the printing paper). Accordingly, in the case of a line head whose head portion cannot move in the line direction since nozzles are arranged in parallel in a width direction, a method such as the first example or the third example cannot substantially be employed. This is because, since the line head does not move in the line direction, the first and third examples cannot cope with a situation in which some nozzles have a defect such as no ejection of droplets.